Cold transient enrichment

ABSTRACT

In an air valve carburetor the air valve is connected by a link to one end of a lever pivoted at the opposite end. Intermediate the link and the pivot point, a hanger extends from the lever to a metering rod to position the metering rod in accordance with air valve position. A thermostat responsive to engine operating temperature and a vacuum break unit control the lever pivot point to provide starting and cold enrichment. A cold transient enrichment device bleeds air into the vacuum break unit to increase cold enrichment during acceleration.

llnie States Ptent 11 1 1111 3,869,528 Mick Mar. 4, 1975 COLD TRANSIENT ENRICHMENT 3,281,130 10/1966 Braun et 61. 261/50 AA 3.284.063 11/1966 Bickhaus et al .1 26l/39 B [75] lnvemori Mount Clemens 3.437.081 4/1969 Mennesson 261/69 R Mlch- 3.677.526 7/l972 Picrlot 26l/DlG. I) Assigneez G l Motors C po t o Mitsuyama Cl ill, r [9 Detroit, Mich. I Primary bvummw-Trm R. Miles Flled: 9 1973 Attorney, Agent. or Firn1-C. K. Veenstra [21] Appl. No.: 343,556

[57] ABSTRACT [57] U CL 261/39 761/39 B 761/50 A In an air valve carburetor the air valve is connected by 6 a link to one end of a lever pivoted at the opposite [51] Int Cl 1/10 end. Intermediate the link and the pivot point. a [58] Fie'ld B 39 A hanger extends from the lever to a metering rod to poh 961/50 sition the metering rod in accordance with air valve position. A thermostat responsive to engine operating [56] References Cited temperature and a vacuum break unit control the lever pivot point to provide starting and cold enrich- UNITED STATES PATENTS ment. A cold transient enrichment device bleeds air .9 1 8/1961 i 261/50 AA into the vacuum break unit to increase cold enrich- 4 1 4-3 8; ment during acceleration. 3.243.167 3/1966 Winkler 261/50 AA 1 Claim, 2 Drawing Figures I! 50 56 22 11 52 0 142 I49 151; 226 152 w 226 26 g3 96 w l w 252 32 i 250 I18 8 5456 i'i H2 0 o a 96 5&

2a g m 202 1 94 .24 28 we 1 98 99 12 at u 83 W so 2} w 1 ,4 l 102 m, '92 99 20 a i It 1 Z6 I TO VACUUM BREAK+ COLD TRANSIENT ENRICHMENT This invention relates to internal combustion engine air-fuel mixture control, and more particularly, to means for enriching the air-fuel mixture when accelerating an engine during its warm-up period.

It is conventional practice to include a vacuum break unit in cold enrichment mechanisms such as automatic chokes. The vacuum break unit is effective to lean the air-fuel mixture from the very rich mixture required for starting to a mixture appropriate during the warm-up period.

During the warm-up period, richer mixtures are required during transients, such as initial acceleration, than during other operating conditions. Thus the prior art includes proposals for accelerating pumps which deliver more fuel at low temperatures than at high temperatures and for vacuum break units which permit the choke to close whenever manifold vacuum decreases below a selected level.

This invention provides a cold transient enrichment device which controls the vacuum break unit. Immediately after the engine starts, or after a suitable time delay determined by a check valve mechanism associated with the vacuum break unit, the vacuum unit leans the air-fuel mixture from the very rich mixture required for starting to the mixture required for most cold oper ating conditions. Thereafter, a thermostat gradually leans the mixture as the engine warms to normal operating temperatures. The cold transient enrichment device controls the manifold vacuum supplied to the vacuum break. When manifold vacuum decreases suddenly during warm-up operation, as upon a sudden in crease in load or sudden acceleration, the cold transient enrichment device temporarily interrupts the vacuum supply and bleeds the vacuum break unit to atmospheric pressure. This permits the thermostat to apply extra enrichment for a predetermined time interval.

The details as well as other objects and advantages of this invention are set forth in the remainder of the specification and are shown in the drawing in which:

FIG. 1 is a sectional elevational view of the carburetor showing the metering linkage; and

FIG. 2 is a side elevational view of the carburetor having parts broken away to show the cold enrichment, vacuum break and transient enrichment mechanisms.

Referring first to FIG. 1, the carburetor has a mixture conduit 12 including an air inlet 14 and a mixture outlet 16 which discharges to the engine. A throttle 18 is disposed in mixture outlet 16 in the usual manner on a throttle shaft 20.

An air valve 22 is disposed in air inlet 14 on an air valve shaft 24. A spring 26 is hooked over the downstream edge 28 of air valve 22 and extends to a bracket 30 to bias air valve 22 to the position shown.

A tang 32 reaches upwardly from air valve 22 and is connected by a link 34 to a diaphragm 36. A chamber 38, formed between the right side of diaphragm 36 and a cover member 40, is connected by a tube 42 to a region 44 of mixture conduit 12 defined between air valve 22 and throttle 18.

A chamber 46, defined between the left side of-diaphragm 36 and a cover member 48, is subjected to substantially atmospheric pressure, present in air inlet 14 and in the air cleaner (not shown), through openings such as 50, 52, and 54. (The air cleaner seats on a rim 56 disposed about the upper portion of carburetor 10.)

In operation, chamber 38 is subjected to the subatmospheric pressure created in region 44 as throttle 18 is opened, and diaphragm 36 acts through link 34 to pull air valve 22 clockwise to an open position. Spring 26 is effective to balance the opening force of diaphragm 36, thereby creating a substantially constant subatmospheric pressure in region 44. By thus establishing a generally constant pressure drop across air valve 22, the area about air valve 22 and thus the rotative position of air valve 22 is determined by and is a measure of the rate of air flow through mixture conduit 12.

A tab 58 extends upwardly from air valve 22 and is connected through a link 60 to one end 62 of a lever 64. The opposite end 66 of lever 64 is pivoted about a pin 68. Intermediate ends 62 and 66, a hanger 70 extends from lever 64 into the carburetor fuel bowl 72. The lower end 74 of hanger 70 has a hook 76 which is received in a recess 78 formed in a metering rod 80.

It may be noted that hanger 70 extends through an opening 82 in the cover 84 for fuel bowl 72. Opening 82 is closed by a slider 86 which shifts horizontally during movement of hanger 70.

Metering rod is disposed in a fuel passage 88 having its lower end 90 disposed to receive fuel from a well 92 formed in the bottom of fuel bowl 72. The upper end 94 of fuel passage 88 has an opening 96 through which fuel is discharged into region 44 of mixture conduit 12. It will be appreciated, therefore, that the fuel in fuel bowl 72 is subjected to a substantially constant metering head from the substantially atmospheric pressure in the upper portion of the fuel bowl to the generally constant pressure in region 44.

A metering jet or orifice 98 is disposed in fuel passage 88 around the tip 99 of metering rod 80. As best shown in FIGS. 2 and 3, metering rod 80 has flat tapered surfaces 100 on opposite sides which, upon reciprocation of metering rod 80 in jet 98, varies the area available for fuel flow through jet 98.

In operation, as air valve 22 opens by clockwise rotation, line 60 rotates lever 64 in a clockwise direction. Lever 64 then lifts hanger 70 to move metering rod 80 generally upwardly and rightwardly in fuel passage 88. Thus as air valve 22 is opened to increase the area available for air flow through air inlet 14, metering rod 80 is shifted to increase the area available for fuel flow through metering orifice 98. By this means, a substantially constant air-fuel ratio may be maintained the precise proportion being controlled by the geometry of tapered surfaces 100 and of the linkage between air valve 22 and metering rod 80.

A spring 102 extends from an annular ledge 104 formed in fuel passage 88 to the lower end 106 of metering rod 80 to take up any slack in the linkage and to load metering rod 80 against jet 98.

It should be appreciated that the thickness of metering rod 80 increases from the end of surfaces 100 most closely adjacent passage inlet 90 to tip 99. Tip 99 is therefore enlarged and assists in discharging fuel from fuel passage 88 as air valve 22 and metering rod 80 are moved to increase air and fuel flow. This offsets the greater inertia of the fuel which otherwise could create a mixture temporarily leaner than desired.

A slot 108 is formed in the end 62 of lever 64. Link 6 0 is connected to lever 64 by having one end 110 disposed in slot 108. A link 112 extends from 110 to an arm 114 of a supplementary lever 116 pivoted at 118.

The opposite arm 120 of lever 116 is connected by a link 122 to one end 124 of an aneroid 126. The opposite end 128 of aneroid 126 is connected to a reciprocable plunger 130 threadedly received by an adjusting screw 132, guided in the bore 134 of an adjustable stop 136, and extending to a diaphragm 138.

A chamber 140 defined between the right side of diaphragm 138 and a cover member 142 is subjected to the manifold vacuum in mixture outlet 16 downstream of throttle 18, while a chamber 144 defined between the left side of diaphragm 138 anda cover member 146 is subjected to atmospheric pressure. The resulting rightward bias on diaphragm 138 is resisted by a spring 148 disposed between the head 149 of adjusting screw 132 and a support 150. The linkage is shown in the position assumed when manifold vacuum is sufficient to overcome the force of spring 148.

When throttle 18 is opened and manifold vacuum drops to a point indicative of the need for an enriched air-fuel ratio, spring 148 moves adjusting screw 132 leftwardly until head 149 engages adjustable stop 136. Link 130 is thus moved leftwardly, and a spring 151, extending between end 120 of supplementary lever 116 and a support 152, moves supplementary lever 116 counterclockwise to move link 122 and aneroid 126 leftwardly. Link 112 is then pulled downwardly to reposition end 110 of link 60 in slot 108, thereby resulting in a shorter lever arm defined between end 110 of link 60 and pivot pin 68. This increases the travel of metering rod 80 through meteringjet 98 for equivalent opening movement of air valve 22 to provide an enriched air-fuel mixture.

When manifold vacuum increases to a point indicative ofa need for a leaner air-fuel ratio, diaphragm 138 forces link 130 and adjusting screw 132 rightwardly until the end 154 of adjusting screw 132 engages support 150. This forces aneroid 126 and link 122 rightwardly resulting in clockwise rotation of lever 116. Link 112 then raises the end 110 of link 60 in slot 108 to increase the lever arm defined between link end 110 and pivot pin 68. This reduces the travel of metering rod 80 through metering jet 98 for equivalent opening movement of air valve 22 to provide a lean air-fuel mixture.

Upon a decrease in ambient air pressure or an increase in ambient air temperature, both indicative of a reduction in air density and consequently a reduction in the mass rate of air flow through air inlet 114 for equivalent volume air flow, aneroid 126 expands forcing link 122 rightwardly and causing clockwise rotation of lever 116. Line 112 then raises the end 110 of link 60 in slot 108 to increase the lever arm defined between link end 110 and pivot pin 68. The increased lever arm reduces the travel of metering rod 80 in metering jet 98 for equivalent movement of air valve 22 to prevent air-fuel mixture enrichment caused by a reduction in air density.

Upon a increase in ambient air pressure or a decrease in ambient air temperature, both indicative of an increase in air density, aneroid 126 contracts. Spring 151 then causes counterclockwise rotation of supplementary lever 116, and link 112 then moves link end 110 downwardly in slot 108 to shorten the lever arm defined between end 110 and pivot pin 68. The shortened lever arm increases the travel of metering rod 80 in jet 98 for equivalent movement of air valve 22 to prevent leaning of the air-fuel mixture due to an increase in air density.

Referring now to FIG. 2, a housing 156 encloses a thermostat 158 subjected to engine operating temperatures for example, by passing air in heat exchange relationship with engine exhaust gases and then through housing 156. Thermostat 158 positions a shaft 160 to which a thermostat lever 162 is secured. A link 164 extends from thermostat lever 162 to an intermediate lever 166 pivotally mounted about a cold enrichment shaft 168.

A vacuum break lever 170 is secured to cold enrichment shaft 168 and has a tang 172 engaged by an arm 174 of lever 166. A link 176 extends from vacuum break lever 170 and is received in a slot 178 formed in the plunger 180 of a vacuum break unit 182.

Vacuum break unit 182 includes a diaphragm 184 biased toward the position shown by a spring 186. A chamber 188, defined between the right side of diaphragm 184 and a cover member 190, is subjected to the manifold vacuum in mixture outlet 16 downstream of throttle 18. A chamber 192, defined between the left side of diaphragm 184 and a cover member 194, is subjected to atmospheric pressure. As soon as the engine starts, or after a suitable time delay provided by known time delay units, diaphragm 184 is pulled rightwardly against the bias of spring 186. A washer 196, secured to diaphragm 184, then pulls a cup member 198 toward the right. This compresses a spring 200 to pull plunger 180 toward the right. Link 176 is thus moved rightwardly to rotate vacuum break lever 170 and cold enrichment shaft 168 in a clockwise direction the degree of rotation being limited by engagement of tang 172 wi h arm 174 when the force exerted by spring 200 is balanced by the force exerted by thermostat 158. The resulting counterclockwise movement ofa cold enrichment lever 202, shown in FIG. 1 and secured to cold enrichment shaft 168, moves pivot pin 68 and lever 64 to increase the lever arm defined between link end 110 and pivot pin 68. This reduces travel of metering rod in jet 98 for equivalent movement of air valve 22 and thus leans the air-fuel mixture after the engine starts.

As thermostat 158 is thereafter warmed during engine operation, thermostat lever 162 is rotated clockwise as viewed in FIG. 2. Lever 162 then acts through link 164 to rotate intermediate lever 166 clockwise. This permits further clockwise rotation of vacuum break lever 170 and cold enrichment shaft 168 under the force imparted by vacuum break unit 182 which thus impart counterclockwise rotation to cold enrichment lever 202 as viewed in FIG. 1. The resulting counterclockwise movement of pivot pin 68 carried by cold enrichment lever 202 on enrichment shaft 168 moves lever 64 with respect to link 60 and thus increases the lever arm defined between link end and pivot pin 68. This increase in lever arm reduces the travel of metering rod 80 in jet 98 for equivalent movement of air valve 22 to lean the air-fuel mixture as the engine warms. This will continue as the engine warms until a tang 204 on vacuum break lever engages an adjustable stop 206.

As thermostat 158 cools after engine operation, thermostat lever 162 is rotated counterclockwise as viewed in FIG. 2. Lever 162 then acts through link 164 to rotate intermediate lever 166 counterclockwise as viewed in FIG. 2. At a selected temperature, arm 174 on lever 166 engages'tang 172 on vacuum break lever 170 to move lever 170, and cold enrichment shaft 168, counterclockwise away from stop 206 as shown in FIG. 2. This imparts clockwise rotation to cold enrichment lever 202 as viewed in FIG. 1, and the resulting clockwise movement of pivot pin 68 shifts lever 64. As lever 64 is shifted, metering rod 80 is raised to provide increased fuel flow for cold start. In addition, the lever arm defined between link end 110 and pivot pin 68 is decreased to provide increased travel of metering rod 80 in jet 98 for equivalent movement of air valve 22 and thus provide an enriched air-fuel mixture for cold operation.

Referring again to FIG. 2, a link 208 extends from intermediate lever 166 and is received in a slot 210 of a fast idle cam member 212. Fast idle cam member 212 has a series of steps 214 or alternatively a smoothly contoured surface engaged by a fast idle adusting screw 216 carried in a tang 218 of a lever 220 pivoted on throttle shaft 20. Another tang 222 on lever 220 engages an arm 224 of a throttle lever 226 secured to throttleshaft 20. A link 228 extends from throttle lever 226 and is received in a slot 230 of an air valve lever 232 secured to air valve shaft 24. A tang 234 on air valve lever 232 carries an idle mixture adjusting screw 236 which engages an arm 238 on fast idle cam member 212.

In operation, as the temperature decreases thermostat 158 rotates thermostat lever 162 counterclockwise to lift link 164 and rotate intermediate lever 166 counterclockwise. Link 208 is then pulled upwardly and toward the right to rotate fast idle cam member 212 clockwise. Fast idle adjusting screw 216 is then received on the high step 214 of fast idle cam member 212 to limit clockwise movement in the throttle closing direction of lever 220, throttle lever 226, and throttle shaft 20.

As throttle lever 226 is rotated counterclockwise during opening movement of throttle 18, link 228 is then pulled downwardly to provide counterclockwise rotation of air valve lever 232 and air valve shaft 24 to assist in opening air valve 22. When needed to unload a flooded engine, throttle 18 can be opened completely. This rotates lever 226 counterclockwise and link 228 then opens air valve 22 partially so that little, if any, vacuum is provided in region 44 and fuel flow is minimized.

As thermostat 158 is warmed during engine operation, thermostat lever 162 and link 164 pull intermediate lever 166 in a clockwise direction, as viewed in FIG. 2. This pushes link 208 downwardly and toward the left to permit counterclockwise movement offast idle cam member 212. Fast idle adjustment screw 216 then may engage a lower step 214 on fast idle cam member 212, permitting further movement of lever 220, throttle lever 226 and throttle shaft in the clockwise or throttle closing direction to reduce engine idle speed.

At high temperatures thermostat 158 positions thermostat lever 162, link 164, intermediate lever 166, link 208, and fast idle cam member 212 whereby arm 238 is engaged by idle mixture adjusting screw 236. This limits movement of air valve lever 232 and air valve shaft 24 in the clockwise or air valve closing direction to prevent an overly rich idling air-fuel mixture.

In the foregoing, the term aneroid has been used to describe a bellows responsive to ambient air temperature and/or pressure. When response to atmospheric pressure alone is desired, an evacuated bellows will be used. When response to both atmospheric pressure and temperature is desired, a gas filled bellows will be used.

It will be appreciated that link 60 and lever 64 are, and other components may be, contoured to provide different lever arms in different linkage positions. This permits tailoring of the metering rod response to air alve movement for increased control over the air-fuel ratios provided at various rates of air flow. In some circumstances, for example, the linkage may be contoured to provide first a decrease and then an increase in the lever arm defined between link end and pivot pin 68 as air valve 24 opens.

The foregoing construction is fully depicted in application Ser. No. 343,553 filed concurrently, and the disclosure of that application is incorporated herein by reference.

As noted above, vacuum break unit 212 is effective to lean the air-fuel mixture after starting. This is satisfactory for steady state operation, but in some instances during acceleration before the engine is fully warmed, enrichment of the air-fuel mixture may be required supplemental to that provided by the mechanism described heretofore. This invention provides a cold transient enrichment device 250 to satisfy that re quirement.

As shown in FIG. 2, cold transient enrichment device 250 comprises a pair of cover members 252 and 254 with a diaphragm 256 clamped therebetween to define a pair of chambers 258 and 260. Cover 252 has a fitting 262 connecting chamber 258 to the manifold vacuum in mixture outlet 16 downstream of throttle 18.

A hole 264 through the diaphragm washers 266 and 268 subjects chamber 260 to the pressure in chamber 258. A flap 270 of diaphragm material acts as a check valve which opens upwardly to permit rapid flow from chamber 260 to chamber 258 but which engages washer 268 to restrict flow from chamber 258 to chamber 260.

When the pressure in chamber .260 equals the pres sure in chamber 258, a spring 272 biases diaphragm 256 to the position shown. Upon a reduction in the manifold pressure in chamber 258, as during deceleration, flap 270 opens to permit flow from chamber 260 to chamber 258, and diaphragm 256 remains in the position shown. Upon an increase in the manifold pressure in chamber 258, as during acceleration, flap 270 closes against washer 268 to restrict flow from chamber 258 to chamber 260, and the higher pressure in chamber 258 biases diaphragm 256 downwardly against spring 272. Diaphragm 256 remains depressed as air flows into chamber 254 due to leakage around flap 270, and the time required for equalization pressures.

between chambers 258 and 260 may be closely controlled to thereby time the return of diaphragm 256 to the position shown.

A stem 274 is secured to diaphragm 256 and carries a valve disc 276 which reciprocates between a pair of valve seats 278 and 280. Valve seat 278 is formed about a bleed 282 open to atmosphere or another controlled pressure source, and valve seat 280 is formed about a passage 284 opening from chamber 258. Both bleed 282 and passage 284 open into a chamber 286 having a fitting 288 connected to the vacuum fitting 290 on vacuum break cover member 190.

During operation at constant or increasing manifold vacuum, valve disc 276 engages seat 278 to close air bleed passage 282 and to permit application of manifold vacuum through fitting 262, chamber 258, passage 284, chamber 286, and fitting 288 to vacuum break unit 182. The corresponding operation of vacuum break unit 182 has been described above.

During operation at decreasing manifold vacuum, as during acceleration, valve disc 276 engages seat 280 to close manifold vacuum passage 284 and to permit air to pass through bleed passage 282, chamber 286, and fitting 288 to vacuum break unit fitting 290 and chamber 188. The resulting increase in pressure forces vacuum break diaphragm 184 and plunger 180 leftwardly. This permits leftward movement of link 176 and counterclockwise rotation of vacuum break lever 170 and cold enrichment shaft under the bias of thermostat 158 applied through shaft 160, thermostat lever 162, link 164, and arm 174 ofintermediate lever 166. The resulting clockwise rotation of cold enrichment lever 202, as viewed in FIG. 1, moves pivot pin 68 clockwise and shifts lever 64 to decrease the lever arm defined between link end ll and pivot pin 68. The decreased lever arm provides increased travel of metering rod 80 in jet 98 for equivalent movement of air valve 22 and thus provides an enriched air-fuel mixture for cold transient operation.

Referring again to FIG. 2, after the time required to equalize pressures across diaphragm 256, spring 272 returns diaphragm 256 and disc valve 276 to the position shown. Disc valve 276 then closes air bleed passage 282 and opens manifold vacuum passage 284 to apply manifold vacuum to vacuum break diaphragm 184. The resulting rightward movement of diaphragm 184, plunger 180, and link 176 imparts clockwise rotation to vacuum break lever 170 and cold enrichment shaft 168. The resulting counterclockwise rotation of cold enrichment lever 202, as viewed in FIG. 1, moves pin 68 and lever 64 to increase the lever arm defined between link end 110 and pivot pin 68. This reduces travel of metering rod 80 in jet 98 for equivalent movement of air valve 22 and thus leans the air-fuel mixture.

It will be appreciated, of course, that cold transient enrichment device 250 and vacuum break unit 182 may be integrated into a unitary package if desired.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows:

l. The combination of a vacuum break unit and a cold transient enrichment device for use on an internal combustion engine having an induction passage for air flow to the engine, a throttle disposed in said induction passage for controlling air flow therethrough, metering means for supplying fuel to the engine in proportion to air flow through said induction passage, and temperature responsive means biasing said metering means to increase the proportion of fuel to air upon a decrease in temperature, said conbination comprising a first pressure responsive diaphragm adapted for connection to said metering means, means defining a chamber on one side of said diaphragm, means adapted for subjecting said chamber to the pressure in said induction passage downstream of said throttle whereby said diaphragm may bias said metering means to decrease the proportion of fuel to air in opposition to the bias of said temperature responsive means, means for subjecting said chamber to atmospheric pressure whereby said diaphragm is ineffective to oppose the bias of said temperature responsive means, each of said atmospheric pressure and induction passage pressure means including a valve seat and being disposed whereby said valve seats face one another, a valve member reciprocable between said valve seats, spring means biasing said valve member to engage the valve seat associated with said atmospheric pressure means and thereby obstruct flow therethrough, a second pressure responsive diaphragm connected to said valve member, said induction passage pressure means subjecting one side of said second diaphragm to the pressure in said induction passage downstream of said throttle, means defining a second chamber on the opposite side of said diaphragm, means including a valved orifice subjecting said second chamber to the pressure in said induction passage downstream of said throttle, said valved orifice permitting a rapid decrease in pressure in said second chamber but a slow increase in pressure, whereby upon a decrease in said induction passage pressure said spring means is effective to bias said valve means into engagement with the valve seat associated with said atmospheric pressure means and said first diaphragm is subjected to said induction passage pressure, whereby for a selected period after an increase in induction passage pressure said second diaphragm is effective to overcome the bias of said spring means and pull said valve means into engagement with the valve seat associated with said induction passage pressure means and said first diaphragm is subjected to atmospheric pressure, and whereby after said selected period said spring means is again effective to bias said valve means into engagement with the valve seat associated with said atmospheric pressure means and said first diaphragm is again subjected to said induction passage pressure. 

1. The combination of a vacuum break unit and a cold transient enrichment device for use on an internal combustion engine having an induction passage for air flow to the engine, a throttle disposed in said induction passage for controlling air flow therethrough, metering means for supplying fuel to the engine in proportion to air flow through said induction passage, and temperature responsive means biasing said metering means to increase the proportion of fuel to air upon a decrease in temperature, said conbination comprising a first pressure responsive diaphragm adapted for connection to said metering means, means defining a chamber on one side of said diaphragm, means adapted for subjecting said chamber to the pressure in said induction passage downstream of said throttle whereby said diaphragm may bias said metering means to decrease the proportion of fuel to air in opposition to the bias of said temperature responsive means, means for subjecting said chamber to atmospheric pressure whereby said diaphragm is ineffective to oppose the bias of said temperature responsive means, each of said atmospheric pressure and induction passage pressure means including a valve seat and being disposed whereby said valve seats face one another, a valve member reciprocable between said valve seats, spring means biasing said valve member to engage the valve seat associated with said atmospheric pressure means and thereby obstruct flow therethrough, a second pressure responsive diaphragm connected to said valve member, said induction passage pressure means subjecting one side of said second diaphragm to the pressure in said induction passage downstream of said throttle, means defining a second chamber on the opposite side of said diaphragm, means including a valved orifice subjecting said second chamber to the pressure in said induction passage downstream of said throttle, said valved orifice permitting a rapid decrease in pressure in said second chamber but a slow increase in pressure, whereby upon a decrease in said induction passage pressure said spring means is effective to bias said valve means into engagement with the valve seat associated with said atmospheric pressure means and said first diaphragm is subjected to said induction passage pressure, whereby for a selected period after an increase in induction passage pressure said second diaphragm is effective to overcome the bias of said spring means and pull said valve means into engagement with the valve seat associated with said induction passage pressure means and said first diaphragm is subjected to atmospheric pressure, and whereby after said selected period said spring means is again effective to bias said valve means into engagement with the valve seat associated with said atmospheric pressure means and said first diaphragm is again subjected to said induction passage pressure. 